Frangible bulb for automatic sprinklers and the like



- Patented Jan. 16, 1940 UNITED STATES FRANGIBLE BULB FOR AUTOMATIC SPRINKLERS AND THE LIKE Arthur Roberts, Bolton, England, assignor to General Fire Extinguisher Company, Providence, R. 1., a corporation of Delaware No Drawing. Application February 7, 1938, Se-

rial No. 189,180. 5, 1936 2 Claims.

degree, nevertheless there attaches to the use of a compressible liquid the disadvantage of avariable operating temperature due to unavoidable variations in the strength of the bulbs.

The object of my present invention is to produce a bulb which can be charged with a relatively incompressible liquid, such as mercury, so

that the bulb will operate within a very small margin of the intended temperature of operation and which will nevertheless shatter into small pieces when the predetermined increase of temperature is attained.

In accordance with my invention, the completed but uncharged bulb is chilled from a red heat, as for example by sudden immersion in hot liquid, or in a bath of molten metal of appropriate composition, or in an air blast. The chilling has the effect of inducing severe compressive stress in'the material at the outer part of the bulb walls and oi inducing severe tensile stress in the material at the inner part of the bulb walls so that the slightest fracture of the bulb at any part is followed immediately by a complete shattering of the whole bulb into small pieces. The quenching process adds to the strength of the glass in known manner and so renders the bulb stronger as a strut. Further, the hardened bulb is less liable to accidental fracture from blows or scratches than is an unhardened bulb for the reason that the whole of the bulb exterior is under compression and only the inner surface is under tension where it is naturally protected from injury.

liquid has been necessary Great Britain November If the bulb were to be chilled from a red heat by immersion in a cold liquid or by exposure to a blast of cold air, the resulting strains could be so severe as to give rise to a tendency for the bulb material to crack; I find the necessary degree of stress for the purpose described can be induced in the walls of the bulb without fear of cracking by proportioning the temperature of the immersion bath or of the air blast to the chemical nature of the glass and to the thickness of the material. In general, I obtain a good result by quenching the bulb from red heat in an oil bath heated to about 250 C. l

In a typical example, a bulb of boro silicate glass of approximately 1 length and diameter and having a wall thickness of about 0.05, when quenched in an oil bath to about 250 C. has the degree of hardening desired for the purpose specified. i

In one application of my invention, I charge the chilled bulb with mercury, leaving space for an air or gas bubble or leaving a vacuous space and then seal the bulb in the usual way. Due to the relative incompressibility of mercury, the size of the gas bubble or of the vacuous space becomes the determining factor inthe temperature of ,operation and bulbs preparedin this way may operate within a very small margin of the intended temperature of operation in spite of the wide difference there may be in the bursting strength of the bulb as compared with the fairly Wide margin between the intended and the actual bursting temperature realized with known bulb. constructions.

I claim:

l. A frangible bulb for an automatic sprinkler or the'like made of glass and having the outer portion of its wall under compressive strain and the inner portion of its wall under tensile strain.

2. A frangible bulb for'an automatic sprinkler or the like made of glass and having the outer surface of its wall under compressive strain and having the inner surface of its wall under tensile strain, ARTHUR ROBERTS. 

